Cigarette filter

ABSTRACT

An open-pore polyurethane having a porosity of at least 50 percent and a density of from 0.1 to 0.5 grams per cubic centimeter was found to be particularly effective as a smoke filter for tobacco products. The polyurethane contains coherent spherical particles of less than 10 microns diameter which are separated by interconnecting interstices.

United States Patent 1 Salyer et al.

[ July 17, 1973 CIGARETTE FILTER [75] Inventors: [vol 0. Salyer, Dayton;Robert T.

Jefferson, West Carrollton, both of Ohio [73] Assignee: MonsantoResearch Corporation, St.

Louis, Mo.

[22] Filed: May 4, 1971 [21] App]. No.: 140,286

[52] U.S. Cl. 131/269, 131/10 R [51] Int. Cl A24b 15/02 [58] Field ofSearch 131/267, 269, 10 R,

[561] References Cited UNITED STATES PATENTS 3,574,150 4/1971 Jeffersonet al. 260/25 3,006,346 10/1961 Golding [31/269 X 2,893,402 7/1959Pinsuti 3,358,695 12/1967 Blakey et al. 131/269 Primary ExaminerRobertW. Michell Assistant Examiner-George M. Yahwak Attorney-Frank D. Shearinand L. Bruce Stevens, Jr.

[57] ABSTRACT 10 Claims, No Drawings CIGARETTE FILTER BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to thefiltration of gases, and more particularly, it is concerned with themanufacture of improved smoke filters for tobacco products such ascigarettes.

2. Description of the Prior Art Previously, porous polyurethanestructures have been obtained as polyurethane foams whose preparationand chemistry are well summarized in the book by J. H. Saunders and K.C. Krisch, Polyurethanes, Interscience Publishers, John Wiley and Sons,N.Y., 1962. Briefly, they are produced by reacting an organicdiisocyanate with organic compounds having at least two activehydrogens, e.g., organic acids, amines, hydroxy compounds includingglycols, and polyhydroxy compounds.

The rigid polyurethane foams known to the art have been generallyprepared from polyols which would afford a highly branched orcross-linked structure. The rigid foams have generally had a closed cellstructure comprising interconnected dodecahedra, usually with over 90percent closed cells having intact membranes or walls. For applicationssuch as filters, demanding an open cell structure, such products areobviously useless since there are few interconnecting passageways. Inaddition, typical rigid foams have shown permanent deformation atpercent deflection in the stress-strain relationship, which has limitedtheir utility in structural applications.

The open-cell prior-art rigid polyurethane foams have generally been lowdensity and consisted of interconnected struts left from the opening upof the dodecahedral cells originally formed at the foaming or blowingstage. When the cells are opened, the walls or membranes serving ascommon walls for adjacent dodecahedra are blown open and the materialreformed as struts.

Any number of products known to the art have been used as a smoke filterfor tobacco products. Some filters are made of paper, and others ofcellulose acetate fibers. Many contain a material such as charcoalbetween two plugs of paper or cellulose acetate. Powdered resins such asnylon have been used in this application, and one product even containsa polyurethane powder between two plugs of cellulose acetate. A filterfor tobacco smoke must be able to absorb various products of combustionthat results from smoking tobacco in the form of cigarettes, cigars,pipes, and the like. At the same time they must permit some of theproducts to pass through the filter to satisfy smokers. It can beappreciated that this balance of properties is of interest due to thegrowing realization of the health hazards involved in smoking, as wellas the evidenceof the presence of carcinogenic materials in tobaccosmoke.

SUMMARY OF THE INVENTION It is one object of the invention to providefilters for the removal of unwanted substances from gases. Anotherobject is to provide a filter material for removing products ofcombustion from tobacco smoke. Yet another object is to prepare a filtermaterial for tobacco smoke that is made of an open pore polyurethanestructure.

These and other objects of the invention are achieved by a smoke filterfor tobacco products which comprises an open-pore polyurethane structurehaving a porosity of at least 50 percent, a density of 0.1 to 0.5 gramsper cubic centimeter, and further comprising coherent sphericalparticles of less than 10 microns diameter separated by interconnectedinterstices.

The open pore polyurethane of this invention is made by a method ofpreparing an open-pore polyurethane structure which comprises (a)preparing separate solutions of polyurethane-forming reactantscomprising (1 a mixture of polyaryl polyalkylene polyisocyanates havingthe formula OCN-Q-CmlQ-NGO I wherein n has an average value of 0.5-2.0,containing about 40-50 percent diisocyanate, the balance being tri-,tetraand pentaisocyanates, having a functionality of about 2.1-3.5 and(2) a polyol having a functionality of at least 3.0, in inert organicliquid diluents which form a homogeneous mixture in which thepolyurethane produced herewith is substantially insoluble, (b) mixingthe solution to yield a homogeneous mixture of the reactants having atotal concentration by weight of 10-3 percent and an NCO/OH ratio of0.90-1 .20, pref erably 0.90-1.05, and ceasing said mixing before theonset of gelation, (c) thereafter maintaining said mixture in aquiescent state while the polyurethane solution gels, and (d) removingsaid organic liquid.

By functionality" of the polyisocyanate is meant the average number ofNCO groups per molecule. The isocyanate groups are convenientlydetermined by the amine equivalent method (ASTM D-1638-67T). Thehydroxyl groups of the polyol are determined by appropriate methods(ASTM D-1638-67T) and usually reported as hydroxyl number, i.e., thenumber of milligrams of potassium hydroxide equivalent to the hydroxylcontent of 1 gram of the sample. The NCO/OH ratio is the equivalentweight of isocyanate groups present in the polyisocyanate reactantdivided by the equivalent weight of hydroxyl groups present in thepolyol reactant.

By homogeneous" is meant a mixture that is essentially uniform. This canbe determined by sampling and. analysis. In some instances it can bedemonstrated by the absence of visible striations characteristic ofpoorly mixed liquids having different refractive indices. Still anothertest is incorporation of a dye or coloring matter in one solution andobservation of the uniformity of dispersion on mixing with a secondsolution.

By gelation" is meant the change of state from the original usuallyclear solution to a gel or jelly, usually opaque. It is readily apparentas a. visible phenomenon or may be detected by suitable viscositymeasurements on segregated portions of the mixture, as with a Brookfieldrotational viscometer, whereby a sharply rising viscosity indicates theonset of gelation.

Unlike the prior art products, the open-pore polyurethane structurescomprise agglomerated coherent spherical particles rather thaninterconnected struts left from blown dodecahedral cells as in foamproducts. The present structures are remarkably uniform and have a highdegree of porosity and can be obtained in a density varying from 0.1 to0.5 gram per cubic centimeter. The structures are compressible and, forcompressions of less than 20 percent, give full recovery. Further thepresent structures may be made in a variety of pore sizes, usually lessthan microns. Greater details on the preparation of these structures maybe found in U.S. Pat. No. 3,574,150 issued Apr. 6, 1971 which isincorporated herewith by reference.

The present material offers several advantages over prior art filtermaterials. The structures can be cast in the proper shape of a tobaccofilter. Because there is practically no shrinkage during gelation,precipitation and drying, the cast structure occupies the same volume asthe homogeneous mix of reactants. The structure can be used directly asthe filter without the necessity of plugs to hold the filter in place.As an example, the structure cast as a rod of the appropriate size canbe rolled in cigarette paper with the tobacco to make a cigarette.Furthermore, the cast structure does not contain a nonporous outer skin,which would make it objectionable for use as a cigarette filter.

Although the material may be cast directly into the shape of the filter,other methods known to the prior art may be used to form the filter. Asan example, the material may be cast into a flat sheet and the filterspunched or cut from the bulk material. The filters, usually in the shapeof cylindrical rods, may be machined to size from a larger piece. Thus,the method of forming the filter shape is not limited to casting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The conditions for formingopen-pore polyurethane structures are set forth in the cited U.S. Pat.No. 3,574,150, issued Apr. 6, 1971, and are incorporated herewith byreference. These include the choice of organic liquid diluent ordiluents, the polyisocyanates, and the polyols; factors such as thepresence of catalysts, the concentration of reactants, the NCO/OH ratioof the system, and the temperature; the necessity for a homogeneoussystem prior to gelation; the necessity for absence of shearing duringgelation; and the possibility of altering the structure by additives,fillers, surfactants, water or pneumatogens.

Preferred polyols are selected from the reaction product of ethylenediamine and propylene oxide having a molecular weight of 274-300 and ahydroxyl number of about 750-800, and the reaction product ordiethylenetriamine and propylene oxide having a molecular weight of400-600 and a hydroxyl number of about 450-800.

The homogeneous solutions of the urethane reactants prior to gelation,containing the urethane precursors, are readily transferred to anappropriate mold to form the filter material. Unlike solutions ofconventional polymers, these solutions have low viscosity and flowreadily even at high concentration of the reactants. They are thereforehandled easily and are adaptable to unusual shapes and structures, ifnecessary. The solution time" prior to gelation is readily establishedby experimentation. Transfer is then made safely during the periodwithout jeopardizing the properties of the polymer structure. Oncegelation occurs the mixture must be kept in a quiescent state to insureoptimum uniformity. If the material is cast as a straight tube, it isreadily filled and left standing in a vertical position until gelationis complete.

Inert organic liquid diluents, e.g., toluene, carbon tetrachloride, andthe like, can be removed by passing a gas such as helium through thematerial, preferably with gently warming. Prior to this step, it may bedesirable to flush the cast material at room temperature with additionalorganic liquid diluent. In this way, uncombined reactants and solublelow molecular weight polymers are removed. Additionally, other methodsknown in the art may be used to remove the inert organic liquiddiluents, such as by the use of vacuum with or without gentle warming.

The invention is further illustrated by, but not limited to thefollowing examples.

EXAMPLE 1 This example illustrates the use of reactants having NCO/OH1.00.

A solution of a polyol was prepared from grams of a product resultingfrom the oxypropylation of diethylene-triamine, having a molecularweight of about 590, a hydroxyl number of about 480 and hydroxylfunctionality of about 5.0 (hereinafter referred to as LA-475)commercially available from Union Carbide Corporation, in 500 grams oftoluene. A second solution of a polyisocyanate was prepared from 116grams of a mixture of polyaryl polyalkylene polyisocyanates obtained byphosgenating the reaction product of aniline with formaldehyde, saidmixture of organic polyisocyanates having the formula wherein n has anaverage value of 0.5-2.0, containing about 40-50 percent diisocyanate,the balance being tri-, tetraand pentaisocyanates, in this instancehaving a functionality of about 2.5 and an equivalent weight of about131 (hereinafter referred to as crude MDI), in 500 grams of toluene. Thetwo solutions were mixed, stirred for less than a minute untilhomogeneous and quickly poured into a mold, where the mixture stoodquietly as polymerization proceeded. About four hours later thepolyurethane produced was removed from the mold and stored overnight ina ventilated area while the toluene evaporated. The structure was foundto have essentially 82 percent porosity, determined on an Air ComparisonPycnometer, Beckman Model 930. The density of the cured product was 0.24grams per cubic centimeter.

The following examples illustrate the use of a range of concentration intoluene. The procedure of Example 1 was used, using the sme startingmaterials, wherein 100 grams of the polyol (LA-475) and l 16 grams ofthe polyisocyanate (crude MDI) were dissolved separately mixed andstirred for not over one minute until homogeneous, poured into a moldand left undisturbed. After about four hours the product was removedfrom 1 the mold and dried by evaporation of the solvent. The

properties of the products are tabulated as follows:

Toluene Total Properties of Product I: Concn. Weight Density Example Wt.g. g./cc. Porosity 12 1584 0.14 90 3 1224 0.18 86 4 17 1054 0.24 82 517.8 1000 0.24 81 6 18.5 952 0.24 81 7 864 0.27 81 8 649 0.38 75Porosity determined determined with a Beckman Model 930 Air ComparisonPycnometer.

The product of Example 3 showed a surface area of 0.5 square meters/gramby the Standard B-E-T method using nitrogen; the product of Example 7showed 0.8 square meters/gram.

From examination of the above tabulation one skilled in the art couldchange readily the density and porosity of the resulting products toobtain an open-pore polyurethane structure having a porosity of at least50 percent and a density of from 0.1 to 0.5 grams per cubic centimeter,by changing the weight percent solvent.

The open-pore polyurethane (OPU) was evaluated as a filter fortobaccosmoke and compared with commercially used materials. The presentmaterial was cut to the appropriate size to replace the existing filterin commercial brands of cigarettes. Smoke from the burning tobacco wasdrawn through the filter at a constant rate of about -32 standard cubicfeet per hour, and the effectiveness of each filter was determined fromthe increase in weight. For comparative purposes commercial cigaretteswere subjected to the same test. The following tabulation shows theaveraged results of the evaluations:

Filter Density Weight Example Material (g./cc.) increase 9 Cellulose0.14-0.17 33.5

Fiber 10 CPU 0. I 74 39.5 1 1 CPU 0. I 70 45 .0 l 2 OPU 0. I 85 42.5 13Polyurethane -0.33 23 .2

Powder 14 Cellulose 0.17 27.5

Fiber 1 Although the invention has been described in terms of specifiedembodiments which are set forth in considerable detail, it should beunderstood that this is by way of illustration only, and that theinvention is not necessarily limited thereto, since alternativeembodiments and operating techniques will become apparent to thoseskilled in the art in view of the disclosure. As an example any numberof solvents may be used in lieu of the toluene solutions describedabove. Flavorings or perfumes may be incorporated in the polyurethane toenhance the aesthetic quality of the filtered cigarette.

Fibrous fillers such as cellulose, and particulate fillers such asactivated charcoal, carbon black, metal oxides and metallic powders canalso be incorporated in CPU for possible enhancement or specialfiltering effects.

5 The incorporation of metallic powders in the CPU has been previouslydescribed in our copending application Ser. No. 54,298 filed July 13,1970, now US. Pat. No. 3,647,721. Furthermore, the CPU can be comminutedto granules and incorporated in a smoke filter between plugs to retainthe granules. Accordingly, modifications are contemplated which can bemade without departing from the spirit of the described invention.

What we calim is:

l. A smoke filter for tobacco products which comprises an open-porepolyurethane structure with a cross section corresponding to that of atobacco smoke passage and having a porosity of at least 50 percent, adensity of 0.1-0.5 grams per cubic centimeter, and further comprisingcoherent spherical particles of less than 10 microns diameter separatedby interconnected interstices, said open-pore polyurethane being thereaction product of (l a mixture of polyaryl polyalkylenepolyisocyanates having the formula wherein has an average value of0.5-2.0, containing about 40-50 percent diisocyanate, the balance beingtri-, tetraand pentaisocyanates, having a functionality of about2.1-3.5, and (2) a polyol having a functionality of at least 3.0.

2. A smoke filter of claim 1 prepared by a method comprising the stepsof i a a. preparing separate solutions ofpolyurethane forming reactantsin inert organic liquid diluents which are capable of forming ahomogeneous mixture in which the polyurethane is substantially insolublecomprising:

1. a solution of a first inert organic liquid diluent and a mixture ofpolyaryl polyalkylene polyisocyanates having a functionality of about2.1-3.5, and containing about 40-50 percent diisocyanate, the balancebeing tri-, and tetra-, and pentaisocyanates, said polyisocyanateshaving the 55 formula wherein n has an average value of 0.5-2.0, and

2. a solution of a second inert organic liquid diluent and a polyolhaving a functionality of at least 3.0 selected from i. the reactionproduct of ethylene diamine and propylene oxide having a molecularweight of 275-300 and a hydroxyl number of about 750-800, and

ii. the reaction product of diethylene triamine and propylene oxidehaving a molecular weight of 400-600 and a hydroxyl number of about450-800;

b. mixing solutions (1) and (2) and making a homogeneous mixture of thereactants having a total concentration of weight of -30 percent and anNCO/OH ratio of 0.90-1.05 and ceasing said mixing before the onset ofgelation;

c. thereafter maintaining the product of step (b) in a quiescent statewhile the polyurethane is precipitated; and

d. removing the inert organic liquid diluents; and

e. shaping the resulting product into the form of a smoke filter fortobacco products.

3. A smoke filter made by the method of claim 2 in which the polyol isan oxypropylated ethylenediamine having a molecular weight of about275-300 and a hydroxyl number of about 750-800.

4. A smoke filter made by the method of claim 2 in which the: polyol isan oxypropylated diethylenetriainine having a molecular weight of400-600 and a hydroxyl number of about 450-800.

5. A smoke filter made by the process of claim 2 wherein said first andsecond inert organic liquid diluents are the same material.

6. A smoke filter of claim 1 prepared by a method comprising the stepsof:

a. preparing a homogeneous liquid mixture of polyurethane-formingreactants having a total concentration by weight of 10-30% and an NCO/OHratio of 0.90-1.05 in an inert organic liquid diluent in which thepolyurethane is substantially insoluble, wherein the reactantscomprise 1. a mixture of polyaryl polyalkylene po1yisocyanates having afunctionality of about 2.1-3.5 and containing about 40-50 percentdiisocyanate, the balance being tri-, tetra-, and pentaisocyanates, saidpolyisocyanates having the formula OCN- CH N00 1 L -TQ- wherein n has anaverage value of 0.5-2.0, and 2. a polyol having a functionality of atleast 3.0 selected from i. the reaction product of ethylene diamine andpropylene oxide having a molecular weight of 275-300 and a hydroxylnumber of 750-800, and ii. the reaction product of diethylenetriamineand propylene oxide having a molecular weight of 400-600 and a hydroxylnumber of about 450-800; by mixing together said reactants and inertorganic liquid diluent; b. ceasing said mixing before the onset ofgelation; c. thereafter maintaining the product of step (a) in aquiescent state while the polyurethane is precipitated; and

d. removing the inert organic liquid diluent; and

e. shaping the resulting product into the form of a smoke filter fortobacco products.

7. A smoke filter of claim 1 which is a cylindrical rod.

8. A smoke filter of claim 1 which is comprised of comminuted granulesof the open-pore polyurethane structure.

9.1n a filter cigarette which comprises a hollow cylinder of paper, afilter in one end of the paper, and tobacco placed in the opposite endof the paper, the improvement which comprises a filter of the open-porepolyurethane structure of claim 1.

10. A method of filtering tobacco smoke which comprises drawing thesmoke from the burning tobacco through an open-pore polyurethanestructure having a porosity of at least 50 percent, a density of 0.1 to0.5 grams per cubic centimeter, and further comprising coherentspherical particles of less than 10 microns diameter separated byinterconnected interstices, said open pore polyurethane being thereaction product of (1) a mixture of polyaryl polyalkylenepolyisocyanates having the formula OCN-Q-OH -iQ-NCO ity of at least 3.0.

2. A smoke filter of claim 1 prepared by a method comprising the stepsof a. preparing separate solutions of polyurethane forming reactants ininert organic liquid diluents which are capable of forming a homogeneousmixture in which the polyurethane is substantially insoluble comprising:2. a solution of a second inert organic liquid diluent and a polyolhaving a functionality of at least 3.0 selected from i. the reactionproduct of ethylene diamine and propylene oxide having a molecularweight of 275-300 and a hydroxyl number of about 750-800, and ii. thereaction product of diethylene triamine and propylene oxide having amolecular weight of 400-600 and a hydroxyl number of about 450-800; b.mixing solutions (1) and (2) and making a homogeneous Mixture of thereactants having a total concentration of weight of 10-30 percent and anNCO/OH ratio of 0.90-1.05 and ceasing said mixing before the onset ofgelation; c. thereafter maintaining the product of step (b) in aquiescent state while the polyurethane is precipitated; and d. removingthe inert organic liquid diluents; and e. shaping the resulting productinto the form of a smoke filter for tobacco products.
 2. a polyol havinga functionality of at least 3.0 selected from i. the reaction product ofethylene diamine and propylene oxide having a molecular weight of275-300 and a hydroxyl number of 750-800, and ii. the reaction productof diethylenetriamine and propylene oxide having a molecular weight of400-600 and a hydroxyl number of about 450-800; by mixing together saidreactants and inert organic liquid diluent; b. ceasing said mixingbefore the onset of gelation; c. thereafter maintaining the product ofstep (a) in a quiescent state while the polyurethane is precipitated;and d. removing the inert organic liquid diluent; and e. shaping theresulting product into the form of a smoke filter for tobacco products.3. A smoke filter made by the method of claim 2 in which the polyol isan oxypropylated ethylenediamine having a molecular weight of about275-300 and a hydroxyl number of about 750-800.
 4. A smoke filter madeby the method of claim 2 in which the polyol is an oxypropylateddiethylenetriamine having a molecular weight of 400-600 and a hydroxylnumber of about 450-800.
 5. A smoke filter made by the process of claim2 wherein said first and second inert organic liquid diluents are thesame material.
 6. A smoke filter of claim 1 prepared by a methodcomprising the steps of: a. preparing a homogeneous liquid mixture ofpolyurethane-forming reactants having a total concentration by weight of10-30% and an NCO/OH ratio of 0.90-1.05 in an inert organic liquiddiluent in which the polyurethane is substantially insoluble, whereinthe reactants comprise
 7. A smoke filter of claim 1 which is acylindrical rod.
 8. A smoke filter of claim 1 which is comprised ofcomminuted granules of the open-pore polyurethane structure.
 9. In afilter cigarette which comprises a hollow cylinder of paper, a filter inone end of the paper, and tobacco placed in the opposite end of thepaper, the improvement which comprises a filter of the open-porepolyurethane structure of claim
 1. 10. A method of filtering tobaccosmoke which comprises drawing the smoke from the burning tobacco throughan open-pore polyurethane structure having a porosity of at least 50percent, a density of 0.1 to 0.5 grams per cubic centimeter, and furthercomprising coherent spherical particles of less than 10 microns diameterseparated by interconnected interstices, said open-pore polyurethanebeing the reaction product of (1) a mixture of polyaryl polyalkylenepolyisocyanates having the formula